The present invention relates to a grounding structure for an electromagnetic wave shield including a substrate formed of a resin material and a coated layer formed by coating the substrate with an electromagnetic wave shielding paint, with a metal plate disposed in abutment under pressure against the coated layer for providing a ground connection.
Electronic and electrical appliances, instruments and systems often act as noise sources which cause, or are exposed to, electromagnetic interference. Accordingly there is a need for a conductive electromagnetic wave shield which is capable of preventing such electromagnetic interference. The development of resin materials and the advances in the molding techniques have created a tendency to replace heavy metal materials with lighter resin materials in electronic and electrical appliances.
An electromagnetic wave shield, which is commonly used to block electromagnetic interference, comprises a substrate formed of an electrically insulated resin material, the surface of which is electroplated or coated with an electromagnetic wave shielding paint to render it electrically conductive. The use of an electromagnetic wave shielding paint as a coating on the surface of the substrate is currently the principal way of providing a conductive coating, in view of its versatility of application depending on the variety of the substrate materials and the complexity of configuration thereof as well as the cost consideration. For example, FIG. 9 shows an electromagnetic wave shield 101 comprising a substrate 102 of a resin material and a coated layer 103 which coats the substrate 102 with an electromagnetic wave shielding paint. A flat metal plate 104 and the shield 101 are secured together as by bolts 105 and nuts 106 so that the metal plate 104 is held in abutment under pressure against the coated layer 103 of the shield 101. The shield 101 and the metal plate 104 are formed with openings 107a and 107b, respectively, to pass the bolts 105 therethrough.
The metal plate 104 is connected to a shield cable 108, whereby the shield 101 is grounded via the metal plate 104 and the shield cable 108. In this manner, the coated layer 103 of the shield 101, the metal plate 104 and the shield cable 108 are electrically connected to each other.
FIG. 10 is a microscopic illustration of the abutment of the metal plate 104 against the coated layer 103. As can be seen, the coated layer 103 partly bites into the metal plate 104. Specifically, the electromagnetic wave shielding paint which forms the coated layer 103 comprises a binder 109 of a resin material and a conductive filler 110 dispersed in the binder 109.
When the metal plate 104 is disposed in abutment under pressure against the coated layer 103, the conductive filler 110 bites into the metal plate 104. Accordingly, when the conductive coated layer 103 and the metal plate 104 are in contact with each other, in the manner shown in FIGS. 9 and 10, the electrical conduction between the coated layer 103 and the metal plate 104 provides a desired conductivity or electromagnetic wave shielding effect.
However, when the assembly shown in FIG. 10 is subject to a thermal cycle (i.e., a heating and cooling cycle), because the metal plate 104 is held in abutment under pressure against the shield 101, rubbing stresses occur between the dissimilar materials of the substrate 102 and the metal plate 104 due to their different rates of thermal expansion. In particular, in a region where an edge of the metal plate 104 is in contact with the coated layer 103 or where a force for holding the metal plate 104 in abutment against the coated layer 103 is concentrated, the coated layer 103 cannot follow the substrate 102, but is exfoliated therefrom, as illustrated in FIG. 11. As a consequence, exfoliation of the coated layer 103 is accelerated under severely changing thermal conditions, as shown in the dotted region in FIG. 12, resulting in poor conduction from point A or C to point B and in a failure to maintain the desired conductivity.
It is an object of the invention to provide a grounding structure for an electromagnetic wave shield and is capable of securing a desired conductivity and suppressing an exfoliation of a coated layer formed on a substrate.